The present invention relates to the formation of surface coatings, and more particularly, to a method for forming on an object a high performance surface coating, characterized by high mechanical strength, exhibiting hardness and wear resistance, stability at high temperatures, and resistance to chemical attacks, and to compositions of the high performance surface coatings.
Currently, there is an on-going search for methods for forming surface coatings characterized by high mechanical strength, exhibiting hardness and wear resistance, stability at high temperatures, and resistance to chemical attacks, and for compositions of such surface coatings. High performance surface coatings are necessary for manufacturing objects used in high performance applications, including devices such as instruments, tools, equipment, machines, components of each of these, and work pieces.
There is a wide range of applications for high performance coated objects, each of which involves one or more forms of extensive or high levels of physicochemical phenomena or interactions taking place at or on the coated surface of a given object, involving an object and a work piece, two objects, an object and its local environment, or a combination of these. These forms include mechanical phenomena such as friction, abrasion, and related mechanisms involving hardness and material wear, thermal phenomena such as heat generation, absorption, and transfer, and chemical phenomena such as oxidation, corrosion, or reaction between materials. Applications which use high performance surface coated objects include, for example, devices, components, and work pieces, involving cutting, grinding, milling, drilling, polishing, rotating, or turning, used in a diversity of industries such as mining, metal working, construction, medical and aeronautical applications, and power generation.
High performance applications also include using coated objects such as rotators, ball bearings, gears, pistons, and blades, in larger objects or devices such as generators, motors, and turbines, used for example, in land, sea, and air vehicles such as automobiles, boats, and airplanes, and for example, in electric power stations.
There is an increasing need for objects with high performance surface coatings, both for effecting processes and as components, in the semiconductor, electronics, and electro-optics industries. An example is the manufacture of electronic components having a high performance coated surface, for maintaining functionality during conditions of high levels of heat radiation or heat absorption, used in larger electronic equipment and instrumentation.
In objects having a high performance coated surface, the active region where the extensive physicochemical phenomena or interactions take place is characterized by the volume encompassing the active or coated surface area extending along an edge or contour of the coating and penetrating into the surface coating to a depth usually of less than 250 microns. But the volume extend to a depth of the order of 1000 microns. This can be considered the high performance surface coated region, and is typically composed of at least one layer of one or more coating materials, formed by one or more methods for coating the surface of the object.
Base materials currently used for manufacturing objects used in high performance applications, upon which surface coatings are formed, include (i) metals alloys such as ferrous and non-ferrous alloys, for example, steel or tungsten alloyed with transition group metals such as cobalt or nickel; (ii) metal carbides such as tungsten carbide, titanium carbide, tantalum carbide, or niobium carbide; (iii) metal nitrides such as titanium nitride, tantalum nitride and hafnium nitride; (iv) metal borides such as titanium boride, tantalum boride, and hafnium boride; (v) cemented carbides featuring a metal carbide cemented by one or more transition group metals such as iron, cobalt, and nickel; (vi) oxide and non-oxide ceramics such as alumina or silicon nitride; (vii) cermets or ceramic metal composites featuring a ceramic bonded to a metal such as transition group metals cobalt, nickel, or molybdenum; and (viii) semiconductor materials and oxides such as silicon or silicon dioxide.
Base materials of high performance objects typically include a relatively small concentration of one or more base material strengthening or enhancing agents, where each strengthening or enhancing agent exhibits at least one particular physicochemical property or characteristic, desirably transferable to the base material, in order to improve object properties, function, and/or in service durability. Base material strengthening or enhancing agents include (i) at least one of the above listed base materials to be used in combination with the base material of the object. Additional base material strengthening or enhancing agents include (ii) non-metal carbides such as silicon carbide, (iii) non-metal nitrides such as silicon nitride and boron nitride, and (iv) metal carbide/nitride mixtures such as titanium carbonitride.
For further enhancing mechanical, thermal, and chemical properties and characteristics of a base material of a high performance object, or of a base material, which includes a strengthening or enhancing agent, a coating of usually less than about 250 microns is formed on the surface of the base material of the object. Typical surface coating materials are selected from any of the above listed base materials, base material strengthening agents, or diamond, and applied to a selected base material surface, using specialized methods and procedures.
A wide variety of methods for forming a coating on an object for a high performance application exists. Such methods, including chemical vapor deposition (CVD), plasma chemical vapor deposition (PCVD), plasma assisted chemical vapor deposition (PACVD), chemical vapor infiltration (CVI), laser techniques, and electrophoretic deposition (EPD), the advantages, scope of application, and limitations of each, are well described and taught in the patent literature.
Limitations exist with respect to forming high performance surface coatings on objects to be used in high performance applications. It is of prime importance that the surface coating be mechanically strong exhibiting hardness and wear resistance, and be thermally and chemically durable, as well as adhering to the surface coating substrate. Moreover, it is important for the surface coating to exhibit these same properties and characteristics under working conditions of a given application. These attributes of coated objects are strongly influenced and limited by the coating method, where each method involves the selection of coating parameters such as particular temperatures required for the coating process, geometrical conformation to the shape of a particular substrate, as well as coating thickness, density, uniformity, and adhesion to a base material substrate. Additionally, the rate of coating formation is an important processing parameter when scaling-up a coating method for inclusion into a time dependent production line for the manufacture of a high performance object.
Using any known method for forming a coating on a surface of an object serving as a substrate typically results in attaining several but not all desirable attributes needed for high performance applications. For example, the rate of coating formation may be high, but the coating may be insufficiently dense. Similarly, a coating may be sufficiently thick or dense, yet the coating may not sufficiently adhere to the substrate surface. Additionally, a rapidly formed coating of sufficient thickness and density may not possess the proper combination of physicochemical properties for high performance in service durability, as described below for the case of a thick diamond coating limited by chemical activity at high temperatures. These are significant limitations with respect to current methods for coating objects for use in high performance applications.
Recently, it has been taught in commonly owned, U.S. Pat. No. 6,258,237, that by employing a method combining more than one coating technique, for example, by using EPD for depositing an initial coating on the base material of an object serving as a substrate, followed by using a second coating technique for further enhancing the properties of the initial coating by depositing onto or infiltrating into the initial surface coating, another coating material exhibiting desirable properties and characteristics, several limitations and difficulties for forming thick and uniform coatings can be overcome. In the aforementioned U.S. Pat. No. 6,258,237, an initial coating is formed on a substrate by electrophoretically depositing positively charged diamond particles on the surface of the substrate, for obtaining a green diamond coating on the surface of the substrate. Following this process, the green diamond particles are consolidated or densified by using another deposition method, such as gas-phase deposition, for example, CVD, chemical deposition, or electrochemical deposition, for depositing a metal throughout interstices of the initial diamond coating, resulting in a thick and uniform diamond layer, adherent to the substrate.
Diamond is commonly known as being the hardest pure material and for exhibiting excellent thermal, electrical, and optical properties. However, there are several limitations in using diamond as a coating material for forming high performance coatings on base material substrate surfaces. For example, diamond coatings are limited to use at temperatures below about 600xc2x0 C. at which temperature diamond is oxidized. Additionally, diamond coatings are chemically and thermally limited during applications involving contact with alloys based on iron, cobalt, and nickel, to temperatures below about 500xc2x0 C., due to chemical reaction with these metals and chemical instability in air.
It is known that an alternative to a diamond coating on an object used in high performance applications is based on using cubic boron nitride (cBN) to form a cBN surface coating on such an object. Cubic boron nitride is a notable material, second only to diamond in hardness. Moreover, cBN is stable in air at temperatures up to about 1200xc2x0 C., in contrast to about 600xc2x0 C. for diamond, and is chemically stable during contact with common metals, including ferrous and non-ferrous alloys such as hard steels, and nickel and cobalt based alloys. These properties and characteristics of cBN extend the range of its application beyond that of diamond. For example, objects manufactured with a cBN surface coating can readily be used for machining steel, cast iron, and other iron, cobalt, or nickel alloys.
A method of coating a surface of a high performance object with cBN involves compaction of a solid cBN plate by hot pressing the cBN at pressures of about 50,000 atm at a temperature below about 1400xc2x0 C. and brazing the plate onto the substrate. The sintering procedure is limited as it must be performed in the thermodynamic stability region of cBN in order to prevent conversion of cubic boron nitride into hexagonal boron nitride (hBN). Moreover, this coating technique has additional limitations with respect to forming a uniform and adherent coating on a substrate surface having specialized or intricate geometry or shape.
Chemical vapor deposition methods such as PACVD and PCVD have been employed for coating the surface of an object using cBN, for example, of base material tungsten carbide, but two significant limitations exist using these gas phase deposition methods. First, in strong contrast to forming diamond surface coatings by the same methods, only very thin coatings of cubic boron nitride are obtained. Second, surface coatings thus formed typically feature a multi-phase mixture of amorphous and hexagonal BN, and cBN, at the coating/substrate interface. These multi-phase boron nitride coatings exhibit poor adhesion to the substrate, thus limiting mechanical strength of the coated object during high performance applications.
To one of ordinary skill in the art, there is thus a need for, and it would be highly advantageous to have a method for forming high performance surface coatings characterized by high mechanical strength exhibiting hardness and wear resistance, stability at high temperatures, and resistance to chemical attacks, and to have compositions of high performance surface coatings, which overcome the above indicated limitations associated with forming and using surface coatings for high performance applications.
The present invention relates to a method for forming on an object a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, and, to compositions of high performance surface coatings. The method is based on a two step process, uniquely integrating a first step of EPD featuring at least one surface coating material for forming a green coating, with a second step of chemical vapor infiltration/deposition (CVI/D) featuring at least one additional surface coating material, for forming on an object a high performance surface coating characterized by high mechanical strength exhibiting hardness and wear resistance, and, thermal and chemical durabilities.
In selected cases of completing the two step process of EPD followed by CVI/D, an optional third step is performed on the obtained surface coated object. The optional third step is heat treating the obtained surface coated object for the purpose of improving coating adhesion to the substrate, and/or for increasing final density by minimizing porosity of the surface coating.
It is therefore an object of the present invention to provide a method for forming on an object a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities.
It is a further object of the present invention to provide a method for forming on an object a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, where the method is applicable to different surface coating materials.
It is a further object of the present invention to provide a method for forming on an object a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, where the method is applicable to different objects featuring different base materials, different base material strengthening or enhancing agents, and different object geometries, shapes, or configurations.
It is another object of the present invention to provide compositions of high performance surface coatings having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities.
It is a further object of the present invention to provide compositions of high performance surface coatings having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, where the compositions feature different surface coating materials.
It is a further object of the present invention to provide compositions of high performance surface coatings having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, where the compositions are applicable to different objects featuring different base materials, different base material strengthening or enhancing agents, and different object geometries, shapes, or configurations.
Thus, according to the present invention, there is provided a method for forming on an object a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, comprising (a) electrophoretically depositing at least one surface coating material on a surface of the object, for obtaining a green coating on the surface of the object; and (b) infiltrating into and depositing onto the green coating at least one additional surface coating material by a gas-phase infiltration/deposition method, thereby forming the high performance surface coating; wherein, at least one of the at least one surface coating material and the at least one additional surface coating material is chemically and physically stable above 800xc2x0 C.
According to further features in preferred embodiments of the invention described below, the step of infiltrating into and depositing onto the green coating the at least one additional surface coating material by the gas-phase infiltration/deposition method is performed in a temperature range of between about 350xc2x0 C. to about 1200xc2x0 C.
According to still further features in the described preferred embodiments, the gas-phase infiltration/deposition method includes chemical vapor infiltration/deposition (CVI/D).
According to still further features in the described preferred embodiments, the method of chemical vapor infiltration/deposition (CVI/D) is selected from the group consisting of non-plasma CVD, plasma assisted CVD (PACVD), and plasma CVD (PCVD).
According to still further features in the described preferred embodiments, the non-plasma CVD is selected from the group consisting of hot wall reactor CVD and laser enhanced CVD.
According to still further features in the described preferred embodiments, the plasma assisted CVD (PACVD) is selected from the group consisting of induction heating PACVD and hot filament PACVD.
According to still further features in the described preferred embodiments, the plasma CVD (PCVD) is selected from the group consisting of DC PCVD, RF induction PCVD and microwave PCVD.
According to still further features in the described preferred embodiments, the step of electrophoretically depositing the at least one surface coating material on the surface of the object includes the step of preparing an EPD suspension including the at least one surface coating material in powdered form, a polar organic liquid, and optionally, at least one additive.
According to still further features in the described preferred embodiments, the at least one additive is selected from the group consisting of a ceramic powder, a pH and conductivity adjusting agent, a charging and surface active dispersing agent, and a binding agent.
According to still further features in the described preferred embodiments, the pH and conductivity adjusting agent is selected from the group consisting of a phosphate ester, acetic acid and hydrochloric acid.
According to still further features in the described preferred embodiments, the charging and surface active dispersing agent is selected from the group consisting of acetylacetone, phosphate ester, and polyacrylic acid.
According to still further features in the described preferred embodiments, the binding agent is selected from the group consisting of menhaden (fish) oil, polyvinyl butyral, polyvinyl alcohol, polyvinyl acid, nitrocellulose and shellac.
According to still further features in the described preferred embodiments, the polar organic liquid is an alcohol.
According to still further features in the described preferred embodiments, the alcohol is selected from the group consisting of ethanol, methanol, and isopropanol.
According to still further features in the described preferred embodiments, the step of electrophoretically depositing the at least one surface coating material on the surface of the object is performed by passing direct electrical current between electrodes of an electrophoretic cell including an EPD suspension featuring the at least one surface coating material, the step of passing the direct electrical current is effected according to a mode selected from the group consisting of constant current and constant voltage.
According to still further features in the described preferred embodiments, each of the at least one surface coating material and each of the at least one additional surface coating material is selected from the group consisting of a high performance base material, a high performance base material strengthening agent, and diamond.
According to still further features in the described preferred embodiments, the surface of the object is selected from the group consisting of a high performance base material and a high performance base material strengthening agent.
According to still further features in the described preferred embodiments, the high performance base material is selected from the group consisting of a metal alloy, a metal carbide, a metal nitride, a metal boride, a cemented carbide, a ceramic, a cermet, and a semiconductor material.
According to still further features in the described preferred embodiments, the high performance base material strengthening agent is selected from the group consisting of a high performance base material, a non-metal carbide, a non-metal nitride, and a metal carbide/nitride mixture.
According to still further features in the described preferred embodiments, the non-metal nitride includes boron nitride of a specific morphologic and crystallographic form, the crystallographic form includes cubic.
According to another aspect of the present invention, there is provided a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, formed on an object by electrophoretically depositing on a surface of the object at least one surface coating material forming a green coating, followed by infiltrating into and depositing onto the green coating at least one additional surface coating material by a gas-phase infiltration/deposition method, thereby forming the high performance surface coating, wherein, at least one of the at least one surface coating material and the at least one additional surface coating material is chemically and physically stable above 700xc2x0 C.
According to another aspect of the present invention, there is provided a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, comprising an electrophoretically deposited at least one surface coating material, formed as a green coating on a surface of the object, and at least one additional surface coating material infiltrated into and deposited onto the green coating by a gas-phase infiltration/deposition method, the coating materials forming the high performance surface coating, wherein, at least one of the at least one surface coating material and the at least one additional surface coating material is chemically and physically stable above 700xc2x0 C.
The method for forming on an object a high performance surface coating having high mechanical strength exhibiting hardness and wear resistance, and, high thermal and chemical durabilities, and compositions of high performance surface coatings according to the present invention serve as significant improvements over existing methods for forming surface coatings, and, of surface coating compositions, needed for manufacturing objects used in high performance applications.
With respect to coating processing conditions and parameters, the method of the present invention is generally applicable to a wide variety of high performance base materials of objects and high performance coating materials. The rate of surface coating by EPD followed by infiltration/deposition is relatively rapid, compared to rates of coating formation in other methods, which is an important consideration when scaling-up a coating method for inclusion into a time dependent production line for the manufacture of a high performance object.
With respect to surface coating quality, properties, and characteristics, a coating thickness of up to at least 100 microns can be formed, where such coatings feature high levels of green density, uniformity, and adhesion to geometrically demanding object shapes and configurations for a variety of widely used high performance base materials. Moreover, coating materials used in the method of the present invention are characterized by being physically and chemically stable above temperatures of 700xc2x0 C., in contrast to significantly lower temperatures of applicability exhibited by coating materials used in prior art methods, for example, only up to 600xc2x0 C. for diamond coatings.